1. Field of the Invention
The invention relates to a method for producing a decorative coating on a glass or glass ceramic substrate and to a glass or glass ceramic substrate produced according to such method. More particularly, the invention relates to a glass ceramic cooktop having a bottom surface coating.
2. Description of Related Art
Glass and glass ceramic substrates with a decorative coating are known from practice. A problem with such coatings is that the layer system is subjected to high thermal loads. When used for induction cooktops, for example, the coatings have to resist temperatures of up to 500° C.
A variety of ways for applying such coatings have already been known from practice.
First, it is known to deposit metallic layers using a PVD process, these layers may in particular be applied by sputtering. Sputtered layers are highly brilliant and have a metallic appearance.
A drawback, however, is that sputtering of metallic layers is quite complex. In particular if it is desired to only partially cover the substrate with a decorative layer, for example in order to mark cooking zones, additional process steps for masking are necessary.
Another drawback is due to the fact that there are only few possibilities to vary the visual appearance of the coating, in particular in terms of the desired color. For example it is very complex to apply black layers using a sputtering process.
Often, another drawback is the conductivity of the layers, which usually makes them not suitable to be arranged in front of capacitive switches.
Also known from practice are decorative coatings based on silicone paints. Although the latter may be applied comparatively easily by screen printing, the mechanical strength of such layers, however, is usually limited, and moreover, discoloration of the coating may be caused under high temperature loads.
Furthermore, it is known to apply decorative coatings by means of glass flux, i.e. enamel coatings. Such enamel coatings have the advantage that by selecting the employed pigments which are added to the glass flux material almost any color can be produced.
However, known glass flux-based coatings can usually only be applied in a limited layer thickness with the resulting opacity and are prone to cracking, which has a significant impact on the visual appearance of the layer. Furthermore, known enamel layers put the employed substrate under tension thus reducing its strength.
In order to prevent cracking and to improve the mechanical strength of the composite material produced, it is known to apply porous enamel layers. However, a drawback of these layers is that the pores are in the range of the wavelength of visible light, so that a matt appearance is caused.
EP 1 435 759 B1 describes a method for applying a decorative coating onto a cooktop, in which a dense layer containing inorganic pigments is applied to the upper surface and a porous layer containing inorganic pigments to the bottom surface. The intention therein is to avoid cracks in the bottom layer by all means.
Mechanically and thermally stable glossy enamel layers for cooktops could not be produced so far.
Also known are coatings produced by a sol-gel process in which at least one pigment or filler is added to the sol-gel material. However, in many cases such sol-gel layers are also prone to delamination or cracking, which is then manifested as a local lightening on the composite material.
Furthermore, it is known to apply luster paints as a decorative coating. A drawback with luster paints, again, is that the selection of the color location is limited and that at least for dark, especially for black coatings noble metal based paints have to be used, which involves increased manufacturing costs.